High-strength steel ubiquitously used in machines, automobiles, bridges, buildings and the like, is produced by, for example, using medium carbon steel such as SCr, SCM or the like specified according to JIS G4104 and JIS G4105, having a C content of 0.20-0.35 wt %, for quenching and tempering treatment. However, it is a well known fact that all grades of steel with tensile strengths exceeding 1300 MPa are at increased risk of hydrogen embrittlement (delayed fracture), and the current maximum strength for architectural steel now in use is 1150 MPa.
Knowledge exists in the prior art for enhancing the delayed fracture resistance of high-strength steel, and for example, Japanese Examined Patent Publication HEI No. 3-243744 proposes the effectiveness of refinement of prior austenite grains and application of a bainite structure. While a bainite structure is indeed effective to prevent delayed fracture, bainite transformation treatment results in increased production cost. Refinement of prior austenite grains is proposed in Japanese Unexamined Patent Publication SHO No. 64-4566 and Japanese Examined Patent Publication HEI No. 3-243745. In addition, Japanese Examined Patent Publication SHO No. 61-64815 proposes addition of Ca. However, testing of these proposed solutions by the present inventors has led to the conclusion that they produce no significant improvement in the delayed fracture properties. Japanese Unexamined Patent Publication HEI No. 10-17985 also discloses hydrogen traps consisting of small compounds, but experimentation by the present inventors has suggested that specific conditions exist on the structures, sizes and morphology of precipitates which exhibit hydrogen trapping functions, and effective hydrogen trapping cannot be achieved based on compound sizes and number densities alone.
Thus, production of high-strength steel with significantly improved delayed fracture properties has been limited in the prior art.